The present invention generally relates to variable resistors, and more particularly to a variable resistor which attenuates an input voltage by an arbitrary amount.
In various electronic circuits, the signal voltage is often attenuated to a desired value. An example of such a process is an attenuation of a signal having a voltage of A V by X dB. In such a process, the accuracy of the amount of the attenuation greatly affects the circuit operation, and an extremely accurate control of the attenuation is required.
FIG. 1 shows a first example of a conventional variable resistor. In FIG. 1, a plurality of resistors (nine in this case) 10 through 18 are connected in series, and a plurality of switches (fourteen in this case) 20 through 33 are coupled between nodes connecting the adjacent resistors and a non-inverting input terminal (+) of an operational amplifier 19 in a tournament connection. The switches 20 through 33 are divided into a switch group A which is made up of the switches 20 through 27, a switch group B which is made up of the switches 28 through 31, and a switch group C which is made up of the switches 32 and 33. The switch groups A, B and C are respectively controlled by control signals S.sub.A, S.sub.B and S.sub.C. Within each switch group, every other switches are turned ON/OFF while the remaining switches are turned OFF/ON, in response to the control signal supplied thereto. For example, the switch 21 is OFF when the switch 20 is ON, the switch 29 is OFF when the switch 28 is ON, and the switch 33 is OFF when the switch 32 is ON.
If the switches 20, 28 and 32 are ON, for example, a divided voltage V.sub.10-11 which is obtained by a series resistor network made up of the resistors 10 through 18 is applied to the operational amplifier through these switches 20, 28 and 32. The divided voltage V.sub.10-11 appears at the node which connects the resistors 10 and 11, and may be described by the following formula (1), where .SIGMA.R.sub.10,,18 denotes a series resistance formed by all of the resistors 10 through 18, .SIGMA.R.sub.11,,18 denotes a series resistance formed by the resistors 11 through 18, and V.sub.IN denotes an input voltage. EQU V.sub.10-11 =(.SIGMA.R.sub.11,,18 /.SIGMA.R.sub.10,,18)V.sub.IN---( 1)
On the other hand, if the switches 27, 31 and 33 are ON, a divided voltage V.sub.17-18 which is obtained by a series resistor network made up of the resistors 10 through 18 is applied to the operational amplifier through these switches 27, 31 and 33. The divided voltage V.sub.17-18 appears at the node which connects the resistors 17 and 18, and may be described by the following formula (2). EQU V.sub.17-18 =(R.sub.18 /.SIGMA.R.sub.10,,18)V.sub.IN ---( 2)
When it is assumed for the sake of convenience that all of the resistors 10 through 18 have the same resistance R, the formulas (1) and (2) can be rewritten as the following formulas (1a) and (2a). ##EQU1##
Other divided voltages V.sub.114 12 through V.sub.16-17 between the divided voltages V.sub.10-11 and V.sub.17-18 can be obtained in a similar manner, and the following relationship can be obtained. EQU V.sub.11-12 =(7/9)V.sub.IN .apprxeq.0.77V.sub.IN EQU V.sub.12-13 =(6/9)V.sub.IN .apprxeq.0.66V.sub.IN EQU V.sub.13-14 =(5/9)V.sub.IN .apprxeq.0.55V.sub.IN EQU V.sub.14-15 =(4/9)V.sub.IN .apprxeq.0.44V.sub.IN EQU V.sub.15-16 =(3/9)V.sub.IN .apprxeq.0.33V.sub.IN EQU V.sub.16-17 =(2/9)V.sub.IN .apprxeq.0.22V.sub.IN
Accordingly, the variable resistor shown in FIG. 1 can vary the attenuation from 0.11 times to 0.88 times depending on the combination of the control signals S.sub.A, S.sub.B and S.sub.C, and an output voltage V.sub.OUT of the operational amplifier 19 can be varied in steps.
However, according to the first example of the conventional variable resistor, the voltage varying width is determined by the voltage dividing width of the series resistor network. For this reason, if an attempt is made to improve the resolution by making the voltage varying width small, the number of required resistors and switches becomes extremely large, and there is a problem in that the circuit scale becomes extremely large.
FIG. 2 shows a second example of a conventional variable resistor. In FIG. 2, a first series resistor group 44 is made up of a plurality of resistors (four in this case) 40 through 43 and functions as an input resistance Rs of an operational amplifier 45. Similarly, a second series resistor group 50 is made up of a plurality of resistors (four in this case) 46 through 49 and functions as a feedback resistance Rf of the operational amplifier 45. The operational amplifier 45 operates as an inverting amplifier and an amplification A.sub.NF thereof is determined by a ratio of the resistances Rs and Rf, that is, A.sub.NF =-Rf/Rs. Switches 51 through 56 are provided with respect to the first and second series resistor groups 44 and 50. Control signals S.sub.s1 through S.sub.s3 and S.sub.fl through S.sub.f3 are supplied to these switches 51 through 56, and all of the switches 51 through 56 are turned OFF or only one switch with respect to each of the first and second series resistor groups 44 and 50 is selectively turned ON in response to the control signals S.sub.s1 through S.sub.s3 and S.sub.f1.
When all of the switches 51 through 56 are OFF, the input resistance Rs has a maximum resistance .SIGMA.R.sub.40,,43 of the first series resistor group 44 and the feedback resistance Rf has a maximum resistance .SIGMA.R.sub.46,,49 of the second series resistor group 50.
If it is assumed for the sake of convenience that all of the resistors 40 through 43 and 46 through 49 have the same resistance R, the maximum resistances .SIGMA.R.sub.40,,43 and .SIGMA.R.sub.46,,49 can both be described by 4R. Hence, the amplification A.sub.NF is -4R/4R=-1. On the other hand, if only the switch 53 provided with respect to the first series resistor group is turned ON, the amplification A.sub.NF is -4R/R=-4.
Accordingly, by appropriately setting the resistances R.sub.40 through R.sub.43 and R.sub.46 through R.sub.49 of the resistors 40 through 43 and 46 through 49, it is possible to switch the amplification A.sub.NF of the operational amplifier 45 in multi-steps depending on the control signals S.sub.s1 through S.sub.s3 and S.sub.f1 through S.sub.f3. In other words, the output voltage V.sub.OUT of the operational amplifier 45 can be varied in steps.
However, according to the second example of the variable resistor, metal oxide semiconductor (MOS) transistors are used for the switches 51 through 56 which are provided with respect to the first and second series resistor groups 44 and 50 in order to obtain a sufficiently high switching speed. As a result, the ONON-resistances of the MOS transistors affect the input resistance Rs and the feedback resistance Rf, and there is a problem in that the amplification A.sub.NF of the operational amplifier 45 becomes inaccurate.
FIG. 3 shows a third example of the conventional variable resistor. In FIG. 3, a first voltage dividing circuit 60 is made up of a plurality of resistors 61 through 65 and switches 66 through 71. This first voltage dividing circuit 60 is used to obtain a volta V.sub.60 by dividing the input voltage V.sub.IN by an ON/OFF combination of the switches 66 through 71. For example, if the switches 66 and 70 are ON and the other switches are OFF, the voltage V.sub.60 becomes a maximum. On the other hand, the voltage V.sub.60 becomes a minimum if the switches 69 and 71 are ON and the other switches are OFF.
A second voltage dividing circuit 80 is made up of a plurality of resistors 81 through 85 and switches 86 through 91. This second voltage dividing circuit 80 varies the ratio of the input resistance Rs and the feedback resistance Rf of an operational amplifier 92 so as to vary the amplification A.sub.NF. For example, if the switches 86 and 90 are ON and the other switches are OFF, the input resistance Rs becomes the series resistance of the resistors 82 through 85, the feedback resistance Rf becomes the resistance of the resistor 81, and the amplification A.sub.NF becomes a minimum because the ratio Rf/Rs becomes a minimum. On the other hand, if the switches 89 and 91 are ON and the other switches are OFF, the input resistance Rs becomes the resistance of the resistor 85, the feedback resistance Rf becomes the series resistance of the resistors 81 through 84, and the amplification A.sub.NF becomes a maximum because the ratio Rf/Rs becomes a maximum.
According to this third example of the variable resistor, the input voltage V.sub.IN can be varied in four steps by the first voltage dividing circuit 60, and the amplification A.sub.NF of the operational amplifier 92 can be varied in four steps by the second voltage dividing circuit 80. For this reason, it is possible to obtain the output voltage V.sub.OUT by adjusting the input voltage V.sub.IN in sixteen (=4.times.4) steps by appropriately switching the ON/OFF combination of the switches provided with respect to the first and second voltage dividing circuits 60 and 80. Compared to the first example of the variable resistor, it is possible to make the circuit scale smaller.
In addition, the current flowing to the second voltage dividing circuit 80 mainly flows through the resistors 81 through 85 and only an extremely small current flows through the switches 86 through 91. Hence, even if MOS transistors are used for the switches 86 through 91, it is possible to suppress the voltage drop generated by the ON-resistances of the MOS transistors. Accordingly, it is possible to accurately adjust the amplification A.sub.NF of the operational amplifier 92, and the problems of the second example of the variable resistor can be suppressed.
However, according to the third example of the variable resistor, the voltage V.sub.60 obtained from the first voltage dividing circuit 60 is variably amplified in the operational amplifier 92 with the amplification A.sub.NF which is set by the second voltage dividing circuit 80. As a result, an offset voltage of the operational amplifier 92 which is added to the voltage V.sub.60 is also subjected to the variable amplification, and there is a problem in that the offset voltage varies depending on a code which is set to control the ON/OFF states of the switches which are provided with respect to the first and second voltage dividing circuits 60 and 80. The code sets the ratio V.sub.IN /V.sub.OUT.
A more detailed description will be given of the offset voltage. Generally, the operational amplifier which is used as a comparator is made up of a differential amplifier circuit formed by a transistor pair having the same characteristic. However, because it is extremely difficult to make a transistor pair having perfectly identical characteristics, an offset voltage is inevitably generated by the difference in the characteristics of the transistor pair. The offset voltage is the voltage which appears at the output when the input of the operational amplifier is zero, and is normally described by a value V.sub.OS which is converted to the input. In other words, it is regarded that a voltage corresponding to the value V.sub.OS is input to the input terminal of the operational amplifier. Therefore, since the regular input voltage in (V.sub.60 in FIG. 3) is inevitably amplified by this value V.sub.OS, there is a problem in that the accuracy of the variable resistor which attenuates the input voltage V.sub.IN to an arbitrary output voltage V.sub.OUT cannot be improved.